Epidemic Community-Acquired Methicillin-resistant Staphylococcus aureus (MRSA) is an emerging infectious disease in previously healthy children and adults without risk factors for MRSA acquisition. These new community-acquired MRSA (CA-MRSA) strains differ from hospital-acquired MRSA (HA-MRSA) strains in that they are often susceptible to non-B-lactam antibiotics, cause disease similar to community-acquired methicillin-susceptible S. aureus (CA-MSSA), and contain the genes encoding the cytolytic toxin, Panton- Valentine leukocidin. When the genetic element responsible for the antibiotic susceptibility patterns in these CA-MRSA strains was characterized, now called "staphylococcal chromosomal cassette mec" type IV or SCCmec type IV, it was realized that a distinct phenomenon had occurred not involving spread of HA-MRSA into the community. Outbreaks of skin and soft tissue infections, necrotizing pneumonia, and sepsis syndromes due to CA-MRSA have been reported with increasing frequency. We are seeking crucial information on how to prevent spread of these strains and how to treat infected and colonized individuals. The natural history of CA-MRSA colonization among household members and its association with infection are issues that must be defined in order to develop effective containment, preventative, and therapeutic strategies. In order to define the transmissibility of CA-MRSA strains, household contacts of an index case with CA- MRSA disease will be tested for colonization by CA-MRSA. Rates of CA-MRSA carriage among household contacts will be compared with rates of HA-MRSA carriage among household contacts of an index patient with HA-MRSA. The frequency with which CA-MRSA strains cause disease will be assessed prospectively by determining the secondary attack rate of disease and this will be compared to that of HA-MRSA. The predominant site of CA-MRSA will also be determined by culturing both the nose and the hand. All strains will be genetically characterized by pulsed-field gel electrophoresis, multi-locus sequence typing, SCCmec typing, and lukS-PV/lukF-PV PCR to determine the clonal repertoire of CA-MRSA strains and assess relatedness among household contacts. The knowledge to be gained from this study is crucial to guide further strategies to deal with the CA-MRSA epidemic. Defining the colonization frequency among household contacts of an index case, the secondary attack rate by CA-MRSA, and the predominant site of carriage will aid in development of containment, preventative, and therapeutic management guidelines.